The present invention relates to fuel cell systems and, more particularly, to a system and method for preparing feed for a fuel processing system which produces the required fuel for the fuel cell system.
Fuel cells typically consist of an anode, a cathode and an electrolyte that separates the anode and cathode. The fuel cell uses a fuel reactant gas, which is typically a hydrogen rich stream, and which is introduced into the anode of the fuel cell, while an oxidant reactant gas, typically air, is introduced into the cathode of the fuel cell. A catalyst in the anode causes the hydrogen to oxidize resulting in creation of hydrogen ions, which pass through the electrolyte to the cathode, thereby creating an electric potential across the fuel cell.
There are different types of fuel cells, namely, polymer electrolyte membrane (PEM) fuel cells, phosphoric acid fuel cells (PAFC), and alkaline fuel cells (AFC), among others.
Fuel processing units, typically various types of reformers, are used to produce the desired hydrogen rich gaseous stream from hydrocarbon fuel. Typically, such reformers require steam along with the fuel for carrying out the desired reaction. In some cases, the fuel processing unit also requires heated air, for example when the fuel processing unit is an autothermal reformer.
FIG. 1 shows a conventional scheme wherein fuel, in this case methane (CH4), is fed to a catalytic steam reformer along with a separate stream of steam which is formed by passing water through a boiler. The catalytic steam reformer reacts the hydrocarbon with water as desired so as to produce a hydrogen rich flow for introduction to a fuel cell.
FIG. 2 shows another known system wherein fuel and steam are prepared and fed to a reformer as discussed above, and also wherein a heated stream of air is provided by passing air through a heat exchanger. The fuel processing unit in this case is either an autothermal reformer (ATR) or catalytic partial oxidizer (CPOX).
Fuel cells are desirable for use in various environments, including transportation and/or vehicular applications, wherein the size of the plant is critical. The need for boilers as illustrated in FIGS. 1 and 2 for heating water to provide steam results in increased space requirements for the power plant. Further, such a water circulation system requires a substantial amount of time to form steam from initial startup, and freezing of such a system is also a concern.
A further drawback of known systems is that the use of a boiler to generate steam renders control of the steam:hydrocarbon ratio difficult, and thereby leads to some instances where excess carbon is present in the reformer, which can result in coking or fouling of the unit.
It is clear that the need remains for improved systems and methods for preparing fuel and fuel constituents for the fuel cell, wherein preferably resulting power plant requires less space.
It is therefore the primary object of the present invention to provide a system and method for preparation of fuel for a fuel cell wherein certain components of conventional systems can be removed so as to reduce space needed for the fuel cell and related components.
It is a further object of the present invention to provide such a system and method wherein control of the ratio of steam to hydrocarbon is facilitated so as to reduce carbon deposition and catalyst deactivation.
It is still another object of the present invention to provide a system and method wherein startup time is reduced and potential problems for freezing water are avoided.
Other objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention, the foregoing objects and advantages have been readily attained.
According to the invention, a fuel cell system is provided, which comprises a fuel cell for reacting a hydrogen rich gas; a fuel processor system for converting a hydrocarbon fuel-steam mixture into said hydrogen rich gas; and means for preparing said hydrocarbon fuel-steam mixture comprising means for superheating a hydrocarbon fuel so as to provide a superheated fuel, and for mixing water with said superheated fuel so as to provide said hydrocarbon fuel-steam mixture.
In further accordance with the present invention, a method is provided for adding steam to a gas stream for a fuel processing system, which method comprises the steps of providing a gas stream; heating said gas stream so as to provide a heated stream having a temperature of at least about 260xc2x0 C.; adding liquid water to said heated stream so as to change said water to steam and provide a gas-steam mixture; and feeding said gas-steam mixture to a fuel processing system.
The system and method of the present invention can be used for providing the desired steam mixed with hydrocarbon fuel, and further for providing humidified air to the reformer in the case of an ATR or CPOX unit, such that the need for a boiler to form steam can be completely avoided, thereby reducing the space occupied by the fuel cell system as desired in accordance with the present invention. Typical fuels would include methane, natural gas, gasoline, Diesel fuel, naphtha or the like.